Not Applicable
The methods and systems relate to charged particle transport and more specifically to measuring beam emittance in a charged particle transport system.
Ion implantation is a standard technique generally used for modifying surface properties of materials. In the semiconductor industry, ion implantation techniques are used for introducing conductivity-altering dopant materials into semiconductor wafers. In a conventional ion implantation system, a desired dopant material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafer. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded into the crystalline lattice of the semiconductor material.
The angle at which the ions forming the beam strike the wafer can determine the extent to which the ions penetrate the wafer. For some applications, it may be necessary to control ion penetration down to the nanometer scale. It can be readily appreciated that even slight deviations in the ion beam axis, or divergence or convergence in the beam itself can result in undesirable results.
Presently, measurements of the divergence or convergence, i.e., angular spread, in the beam at the wafer may not be able to be obtained for some implantation equipment. Thus, confirmation of ion implantation within desired specifications or equipment performance monitoring might not be possible. Further, there may be no way to measure directly the mean direction of the beam with respect to the beamline axis, which can determine the actual implant angle on the wafer. Tilt axis settings available on some equipment, if calibrated properly, may establish these angles with respect to the beamline axis only. However, any deviation of the beam from this direction constitutes an error in the implant angle.
A method for determining beam emittance at a plane in a charged particle transport system, comprises measuring the beam current reaching a current sensor as a moving, straight-edged mechanism traverses the beam in a plane upstream of the sensor; numerically determining a derivative of the measured beam current with respect to the position of the mechanism; determining if there is a position of the mechanism at which the range of angles in the portion of the beam reaching the current sensor falls within the angle subtended at the current sensor by the two edges of the mechanism; determining, based on the derivative, a beam current density xcfx81 corresponding to the beam emittance, when there is a position of the mechanism at which the range of angles in the portion of the beam reaching the current sensor falls within the angle subtended at the current sensor by the two edges of the mechanism; and determining, when there is no position of the mechanism at which the range of angles in the portion of the beam reaching the current sensor falls within the angle subtended at the current sensor, the beam current density by performing curve fitting techniques on the derivative of the beam current profile over a range of positions for which the mechanism intercepts beam current that would otherwise reach and be counted by the current sensor.
In one embodiment, the derivative is given by:             ⅆ      I              ⅆ      x        =                    ρ                  left          edge                    *              ⅆ                  θ                      left            edge                                -                  ρ                  right          edge                    *              ⅆ                  θ                      right            edge                              ,      
where       ⅆ          θ              left        edge              ⁢      xe2x80x83    ⁢  and  ⁢      xe2x80x83    ⁢      ⅆ          θ              right        edge            
are the angles subtended by the downstream current sensor to the left and right edges of the mechanism, respectively, and       ρ          left      edge        ⁢      xe2x80x83    ⁢  and  ⁢      xe2x80x83    ⁢      ρ          right      edge      
are the current densities in the beam passing left and right edges, respectively, of the mechanism, as it moves to the right through the beam. I is the current measured by the downstream sensor, and x is the position of the mechanism in the direction of its travel.